Introduction to Health Physics: Fourth Edition - Ruang Baca FMIPA UB

708 CHAPTER 13
a comparable basis, it is reasonable, for purpose of cancer prevention and ALARA
planning, to limit combined exposure to the sum of the radiation and chemical
exposure. If we make the very reasonable assumption that 1 PEL-hour of chemical
exposure is equivalent in carcinogenic potential to 1 DAC-hour of radiation,
which corresponds to a dose of 2.5 mrems (25 μSv), then we can add the mrem
(or μSv) equivalent of the chemical exposure to the radiation dose to obtain the
equivalent combined dose. For example, consider the case of worker in a radioisotope
laboratory where the ambient radiation level is 0.3 mrem/h (3 μSv/h), and
there is benzene in the air at a concentration of 6 ppm. The PEL for benzene listed
by Occupational Safety and Health Administration (OSHA) in 29 CFR 1910.1000
is 30 ppm. For occupational safety purposes, the equivalent combined dose may be
calculated as:
6 ppm
mrem
D (combined) =
× 4h× 2.5mrems + 0.3 × 4h
30 ppm/PEL PEL -h h
= 3.2 mrems.
SOURCE CONTROL
The rise in worldwide terrorism led to a great deal of concern that some of the
licensed widespread radioactive sources that are being used for peaceful purposes
might be stolen or diverted to a terrorist group for use in a “dirty bomb,” which
refers to a high-explosive bomb that is packed with highly radioactive material.
The dirty bomb actually is a radioactivity dispersal device. Detonation of the bomb
results in the dispersal of the packed radioactivity, and thus creation of an area
of high-level contamination that could lead to radiation casualties as well as to a
terrorized public. Because of the international aspect of terrorism and the worldwide
distribution of radioactive sources for peaceful uses, the International Atomic
Energy Agency (IAEA) took the initiative in preventive action by formulating and
adopting a Code of Conduct in the Safety and Security of Radioactive Sources in 2004.
The Code offers guidance on actions to be taken by the member states to prevent
the theft or diversion of legally licensed sources into the hands of unauthorized
persons for malicious use to inflict harm on individuals, society, or on the
environment.
The Code places all authorized sources into one of five categories, depending
on the radionuclide, the quantity of radioactivity, its intended peaceful use (industrial
radiography, radiation therapy, radiation sterilization, etc.), and the danger
level posed by that source. A dangerous source is defined by the IAEA as “a
source that could, if not under control, give rise to exposure sufficient to cause
severe deterministic effects.” The five categories as defind by the Code are the
following:
Category 1—This category consists of sources that are personally extremely dangerous.
Being close to an unshielded category 1 source for a period of several
minutes to an hour would probably lead to death.